Monitoring a network path

ABSTRACT

The present invention relates to the monitoring of paths along which signals are carried in a telecommunications network, in particular to determine if a path has been altered. Each path has a transit time associated therewith for data transport along that path. The method comprises the steps of: monitoring the difference between the transit time of a first signal path and the transit time of a second signal path, such that a change in the difference between the transit times of the two paths can be detected; and, in dependence at least in part on any such detected change, generating an alarm signal. The alarm signal can be used to indicate to a customer that one of the paths has been re-routed.

This application is the US national phase of international applicationPCT/GB2004/003781 filed 2 Sep. 2004 which designated the U.S. and claimsbenefit of GB 0321342.8, dated 11 Sep. 2003, the entire content of whichis hereby incorporated by reference.

FIELD OF TECHNOLOGY

The present invention relates to the monitoring of paths along whichsignals are carried in a telecommunications network, in particular todetermine if a path has been altered.

BACKGROUND

It is known for a network operator to provide a customer with two pathsover which the customer's content data can be transmitted. Thus, if onepath is interrupted or inadvertently broken, content can still bereceived by the customer's intended recipient over the other path. It ispreferable for the two paths to be “diverse”, such that the paths followa different geographical route. This reduces the risk that the pathswill be interrupted together. However, a network operator may re-routeone or both of the paths without the customer necessarily being aware.Such a change in path can easily be accomplished by a network operator,for example by dynamically re-configuring the routers along a path suchthat data is sent over different links.

SUMMARY

According to one aspect of the invention, there is provided a method ofdetermining if one or more of at least two signal paths has beenaltered, each path having a transit time associated therewith for datatransport along that path, the method comprising the steps of:monitoring the difference between the transit time of a first signalpath and the transit time of a second signal path such that a change inthe difference between the transit times of the two paths can bedetected; and, in dependence at least in part on any such detectedchange, generating an alarm signal.

A change in the transit time of one of the paths is likely to indicatethat the path has been re-routed, since re-routing will normally affectthe length of a path. Likewise, other alterations to a path such as thesubstitution of one transport medium such as optical fibre with anothertransport medium such as copper is likely to also affect the transittime. Thus, the alarm signal can be used to notify a customer of thelikely alteration to a path. It can be important for a customer to bealerted to a change in path for several reasons. For example, the newpath may include a link or other equipment which is known to beparticularly vulnerable to faults or damage. Alternatively, as a resultof re-routing, paths which were originally diverse may coincide over atleast part of their lengths. Paths may coincide where the first andsecond signal streams are carried over the same data link, for examplethe same optical fibre, or where the first and second signal streams arecarried by different fibres, cables or other links located within thesame duct. However, in a region where the paths coincide, anyinterruption in that region is likely to affect both paths, which can beundesirable.

Because the difference in the transit times between the paths ismonitored, a customer may not know which of his paths has been altered.However, it is easier to measure a difference in transit times than itis to measure the actual transit time of a path.

The two paths may each extend between a common upstream location and acommon downstream location so as to provide a fault tolerant system fortransmitting content between the two locations. Only one path at a timemay be used for transmitting the content, but preferably each path willbe used concurrently to carry data representative of the same content.The data on each path will preferably be representative of the contentto the same level of quality, but alternatively to conserve bandwidth,one path may be used as a reserve path, carrying data representative ofthe content to a lower level of quality than that of the other path.Alternatively, the reserve path may carry a subset of the data carriedby the main path. The content carried by the paths will preferably bevideo content, it being important for such content to be transmitted ina fault-tolerant manner, the percentage down-time normally expected fortransmission of such data being as low as 0.02%. However, the contentmay be other data such as audio data which can require delivery with avery low down-time.

To determine the difference in the transit times associated with thepaths, marker signals may be introduced into each of the paths.Preferably, the marker signals will be dispersed between the contentdata carried by the paths. If the content data is carried as a stream ofpackets, the marker signals may be introduced into empty packets locatedwithin the streams.

The marker signals will preferably be introduced into the respectivepaths such that for each marker introduced into one path, acorresponding marker is introduced into the other path. In this case,the difference in the transit times of corresponding markers, and hencethe difference in transit time associated with each path, can bemonitored.

The markers may contain correspondence information indicating whichpairs of markers correspond to one another, to make it easier for thetransit time of corresponding markers to be determined. However,correspondence information may not be required if corresponding markersare introduced into each path substantially simultaneously and withsufficiently long time intervals between respective pairs of markers.

The marker signals may each include a time stamp. The respective timestamps of markers may indicate the time difference between the launch ofcorresponding markers onto each path, possibly with correspondenceinformation which can be used at the downstream location to determinewhich markers correspond to one another. However, in a preferredembodiment, a common clock source will be provided at the upstreamlocation such that each marker can include a time stamp indicative ofits time of departure relative to a common time reference at theupstream location.

Preferably, the arrival times of markers at the downstream location willbe monitored. The difference in the arrival times may then be useddirectly or indirectly to monitor the difference in the transit times ofmarkers travelling along the two paths.

A common clock source may be provided at the downstream location formonitoring the arrival times of markers from each of the paths relativeto a common reference. The common clock source at the downstreamlocation may be formed from a single clock or counter. Likewise, thecommon clock source at the upstream location may be formed by anotherclock or counter. Alternatively, one or each of the clock sources may(each respectively) be formed by a pair of clocks, one for each path,provided that the clocks within each pair can be operated such that theydo not drift excessively relative to one another.

However, it will be appreciated that on the one hand it may be importantto know the relative time of departure of markers on the first path asmeasured relative to that of markers on the second path, and that on theother hand it may be important to know the relative time at which markersignals arrive along one path at the downstream location as measuredrelative to the time at which marker signals arrive along the other pathat the downstream location. Therefore, an upstream clock source may beused which is not necessarily synchronised to the downstream clocksource, and vice-versa. It will be particularly advantageous to be ableuse respective clock sources at the upstream and downstream locationsthat are independent from one another, since when the two locations areseparated by a long distance, for example at least 10 km or 100 km, suchdistances can make it difficult to synchronise the upstream anddownstream clock sources.

Preferably, markers will be introduced onto a network at a sendingstation, which sending station will receive content data from a customerfor onward transmission onto the network. Likewise, the marker signalswill preferably be received at a receiving station, where the differencein transit times between the markers on each path will be evaluated. Thesending station and the receiving station will preferably each bereleaseably connectable to a network. Once a network operator has agreedto provide duplicate paths for a customer, the customer can have thesending station and the receiving station installed at the locationswhere the paths respectively begin and end, so as to monitor the paths.Preferably, a marker signal in one stream will include an indication ofthe time at which that marker signal was introduced relative to the timeat which a marker was introduced into the other stream. That is, themarkers can be introduced in pairs, one marker of a pair in one path,the other marker of the pair in the other path. One marker from eachpair will then preferably have an indication of the time differencebetween the time of insertion of the two markers of a pair, which timedifference can be taken into account when monitoring the difference intransit times.

Since the difference in transit times can be monitored at the downstreamlocation, there is provided, according to another aspect of theinvention, a receiving station which is connectable to a plurality ofpaths, each path having a transit time associated therewith for datatransport along that path, the network station having means for:monitoring the difference between the transit time of a first path andthe transit time of a second path such that a change in the differencebetween the transit times of the two paths can be detected; and, independence at least in part on any such detected change, generating analarm signal. Further aspects of the invention are specified in theappended claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be further described by way of example, withreference to the following drawings in which:

FIG. 1 shows apparatus for determining if a path through a networkportion has been altered according to the present invention;

FIG. 2 shows the network portion of FIG. 1 but where one of the pathshas been altered; and,

FIG. 3 shows a more detailed view of a sending station shown in FIG. 1.

DETAILED DESCRIPTION

In FIG. 1, there is shown a network 10 which is configured to provide afirst path 12 a and a second path 12 b, each of which connects a sendingstation 16 to a receiving station 18. Each path 12 a, 12 b includes aplurality of links 22 formed for example by optical fibre, which linksare connected by a plurality of nodes or routers 24 (the network 10 willnormally include further links and routers which in this example are notshown). The first and second paths 12 a, 12 b respectively originate ata first network input 30 a and a second network input 30 b, which inputs30 a, 30 b are connected to the sending station 16 to allow the sendingstation to send data over the paths 12 a, 12 b. Likewise, the paths 12a, 12 b respectively terminate at first and second network outputs 36 a,36 b, which outputs are connected to the receiving station 18 to allowdata sent over the paths to be received by the receiving station 18. Thepaths are arranged such that data travelling along one path does notshare a link (nor, preferably, a router) with data travelling along theother path. This arrangement reduces the risk that a fault will causeboth paths to fail at the same time.

A customer wishing to send content 25 such as video or audio over thenetwork 10 will provide the data at an input 26 of the sending station.The sending station 16 has a copying stage 28 which copies the contentfrom the customer, the content being passed to the first input 30 a ofthe network as a first stream of data 32 a for transmission over thefirst path 12 a, whilst the same content is passed to the second input30 b of the network as a second stream of data 30 b, for transmissionover the second path 12 b. (Alternatively, the customer may provide theduplicate signal streams, in which case the copying stage will not berequired). The first and second streams of data are respectivelyreceived by the receiving station 18. The receiving station has aswitching element 40 for selectively connecting one of the two paths 12a, 12 b to an output 42 of the receiving station 18, such that thesignal stream from the connected path can be passed to the customer'sintended recipient. The signal stream received from the other path isnormally redundant when both paths are operational. However, if a faultis detected in the connected path by a detector circuit 44 coupled tothe switching element 40, the detector circuit generates a fault signalwhich causes the switching element 40 to selectively connect the output42 of the receiving station 18 to the other (previously redundant) path.In this way, the content data input at the receiving station 18 cancontinue to be received at the receiving station output 42 even if oneof the two paths experiences a fault.

It is possible for the network operator responsible for the network 10to re-route one or both of the paths 12 a, 12 b. For example, the linkconnecting nodes A and B in the first path 12 a may become faulty, inwhich case the first path 12 a can be re-routed through an intermediatenode such as node C, using links (shown in dotted lines) previouslyunused in the first path. Alternatively, the first path may be re-routedas indicated in FIG. 2, such that the link between nodes D and Epreviously used only by the second path is now used by both paths, suchthat data from the first and second streams 32 a, 32 b travel over acommon link.

To determine whether one or both of the paths 12 a, 12 b has beenre-routed, the sending station 16 is provided with a marking stage 50 tointroduce markers into each of the datastreams 32 a, 32 b. For eachmarker introduced into one stream, a corresponding marker is introducedinto the other stream; that is, the markers are introduced in pairs, amarker from each pair being introduced into each respective path (ifthere are more than two paths, for example three paths, markers can beintroduced in triplets). The markers contain correspondence informationwhich can be read by a reading stage 52 in the receiving station 18, sothat the reading stage can determine which markers correspond to oneanother in the respective streams. The reading stage 52 is configured tocompare the arrival time of a marker in one stream with the arrival timeof the corresponding positioned marker in the other stream, and usingthe difference in the arrival times, determine the difference in thetransit times of correspondingly positioned markers travelling betweenthe sending and receiving stations in the respective first and secondstreams.

If one of the paths has changed, as indicated for example in FIG. 2,this will cause the markers travelling along the re-routed path toarrive at the receiving station at a different time to that at which themarkers would have arrived had the path not been changed. Since theother of the two paths (the second path 12 b in FIG. 2) has not changed,the difference in the transit time of corresponding markers will change.If the difference in the transit times of corresponding markers exceedsa threshold value, the reading stage is configured to generate an alarmsignal to alert the customer that a path may have changed. Clearly, if apath is re-routed in such a way that the path length does not change,the transit time of markers in that path is likely to remain the same,in which case no alarm signal will be generated. Likewise, if both pathsare lengthened by the same amount, an alarm signal is unlikely to begenerated.

The threshold value beyond which an alarm is generated will preferablybe chosen in dependence on the fluctuations (either expected ormeasured) in the difference in transit times of corresponding markers inthe two paths when the paths do not change. Typically, the thresholdwill be set to +/−100 micro seconds. This compares with a typicaldifference in the transit times of a few ms, for example 3 ms where twopaths over optical fibre differ in length by 600 km.

If pairs of corresponding markers can each be introduced in a respectivestream substantially simultaneously (that is, such that the timedifference is smaller that the smallest expected change in transit timethat is likely to result from a change in path routing), it can beinferred that a path has changed simply on the basis of a change in thedifference in the arrival times of corresponding markers. However, inpractical situations, there is likely to be a significant offset in thetimes at which corresponding markers can be introduced in the respectivestreams. In such a situation, one or each of a pair of correspondinglypositioned markers will contain information (introduced in the markersby the sending station) which can be used by the reading stage 52 of thereceiving station 18 to determine the difference in the times at whichcorresponding markers were introduced into their respective streams.

FIG. 3 shows a more detailed view of a sending station 16 such as thatof FIG. 1, which in this example is arranged to send video signals overthe network 10 (like components have been given the same numerals). Inuse, a customer's content data 25 (here video data) is introduced at theinput 26 of the receiving station in digital form and passed to thecopying stage 28 (alternatively, the copying stage 28 may include ananalogue to digital converter 29, in which case the customer's contentdata will be entered at the input 26 in analogue form). The copyingstage 28 provides a first stream of digital data 32 a and a secondstream of digital data 32 b. To allow the sending station 16 to transmitvideo signals efficiently, the sending station includes a data encoder49 for compressing and packetising the first and second datastream 32 a,32 b from the copying stage. An encoding stage 51 is provided whichencodes the incoming datastreams according to the MPEG standard, and apacketising stage 53 is provided to packetise the data according to theASI standard (other standards such as Asynchronous Transfer Mode (ATM)could be employed instead).

After being encoded and packetised, the signal streams 32 a, 32 b arepassed to the marking stage 50. The marking stage 50 includes a mastermarker stage 57 a for inserting stamps or other markers into the firstsignal stream 32 a, whilst a slave marker stage 57 b is provided forinserting markers or stamps into the second stream 32 b. The markerstage 57 a has a free running counter or “clock” such as a Relative TimeStamp counter 62 which is set to generate a flag signal at regular timeintervals, in this example every second. The master and slave markerstages 57 a, 57 b are coupled, such that each flag signal generated bythe master maker stage 57 a is passed to the slave marker stage 57 b.Each marker stage 57 a, 57 b is configured to detect in the signalstream the presence of “null” packets whose payload is empty. Inresponse to a flag signal, the master marker stage 57 a is configured toinsert a stamp into the first available null packet present in the firststream. Likewise, the slave marker stage 57 b is configured to introducea corresponding stamp into the second signal stream in response to theflag signal. Packets of encoded video data with marked packets dispersedtherebetween are output at first and second outputs 71 a, 71 b, whichoutputs are releasably connected to the first and second network inputs30 a, 30 b.

Because it is unlikely that a null packet will be available in eachstream simultaneously, stamps are likely be inserted into the two signalstreams at different times in response to a given flag signal. So thatthis offset can be accounted for by the receiving station 18, the mastermarker stage 57 a is configured to read the value of the RTS counter 62at the moment when the master marker stage 57 a is about to introduce astamp into a null packet of the first signal stream 32 a, such that thestamp contains the RTS counter value or “time” at which the stamp wasintroduced, rather than the time at which the flag signal was generated.Likewise the slave marker stage 57 b reads the RTS counter value 62 suchthat when inserting a stamp, it inserts the actual RTS counter value atthe time of insertion, such that any offset in the insertion times forthe two streams can be calculated from the respective RTS values.

Returning to FIG. 1, the reading stage 52 of the receiving station 18 isprovided with a clock stage 64 for recording the time at which markedpackets are received. A processing stage 66 is provided for determiningthe difference in the transit times between marked packets that havetravelled along the first path and the transit time of packets that havetravelled along the second path. Thus, if S1 is the “time” at which apreviously null packet in the first stream is marked or stamped, A2 isthe time at which that packet is received at the receiving station 18,and likewise S2 and A2 are the times at which a corresponding packet isstamped and received respectively in the second stream, then theprocessing stage calculates respective values for (S1-S2) and (A1-A2).The processing stage 66 then calculates the difference between therespective values for (S1-S2) and (A1-A2). In this way, the processingstage simply determines the difference in arrival time (A1-A2), takinginto account any offset (S1-S2) in the time at which stamps areintroduced into marked packets. Clearly, if respective marked packetscan be introduced into each stream with a sufficiently small timeseparation, then (S1-S2) can be neglected, and only the difference inarrival times (A1-A2) need be considered.

The processing stage 66 will be configured to: determine the differenceD between the transit times along the two paths: i.e. to determine thevalue D=|(A1−A2)−(S1−S2)|=|(A1−S1)−(A2−S2)| for corresponding markers(vertical bars indicating the modulus of the quantity between the bars);to determine if D exceeds a threshold value; and, if D exceeds thethreshold value for a predetermined number of consecutive pairs ofcorresponding markers, to generate an alarm signal. Alternatively, theprocessor stage 66 may be configured to determine the average value forD over a period of time, and to generate an alarm signal if the averageexceeds a threshold.

The processing stage 66 is configured to determine the differencebetween the transit times of corresponding markers along the two pathsby using the difference in arrival times A1, A2 of the markers. Thedifference in arrival time can be used directly, in which case thequantity (A1−A2) is evaluated, and a correction due to any offset in thedeparture times (S1−S2) of a pair of markers is made. Alternatively, thedifference in arrival time can be used indirectly, in which case a firstquantity (A1−S1) and a second quantity (A2−S2) are each evaluated,following which the difference between first and second quantitiesobtained. Clearly, in each case is the difference between the arrivaltimes along the two paths that is important, in particular if (S1−S2) isa small correction to the value (A1−A2).

At least one processor 101 is provided at the receiving station on whichcomputer algorithms such as the reading stage, including the processorstage and/or the clock stage can be run in use. Likewise, at least oneprocessor 102 is provided at the sending station on which some or all ofsoftware stages for copying, a/d conversion, marking, encoding, and theRTS counter can be run.

Clearly, the invention provides a simple way for the customer of anetwork operator to find out if paths the customer has been allocatedhave been altered, in particular where the start and end points of thepaths are located sufficiently far away for clocks at the start and endlocations of the paths to be synchronised.

1. A method of determining whether one or more of at least two signalpaths has been altered, the paths each having a transit time associatedtherewith, the method comprising: monitoring the difference between thetransit time of a first signal path and the transit time of a secondsignal path such that a change in the difference between the transittimes of the two paths is detected; comparing the detected change indifference against a threshold value; and in dependence at least in parton any such detected change exceeding the threshold value, generating analarm signal.
 2. A method as claimed in claim 1, wherein the differencebetween the transit times of the paths is monitored by: introducingmarker signals onto the first and second paths at respective entrypoints; receiving the marker signals at respective collection pointsalong the first and second paths; and, monitoring the arrival times ofthe marker signals in one path relative to the arrival times of markersignals in the other path.
 3. A method as claimed in claim 2, whereinfor each marker signal introduced onto one path, a corresponding markersignal is introduced onto the other path, and wherein the difference inthe arrival times of corresponding marked signal is used to monitor thedifference in the respective transit times associated with the first andsecond paths.
 4. A method as claimed in claim 3, wherein the differencein the time of arrival of marker signals is monitored.
 5. A method asclaimed in claim 2, including the step of introducing into each markersignal an indication of the relative time at which that marker signalwas introduced onto a path, the relative time being measured relative toa clock source.
 6. A method as claimed in claim 2, wherein a markersignal in one path includes an indication of the time at which thatmarker signal was introduced relative to the time at which a marker wasintroduced into the other path.
 7. A method as claimed in claim 1,wherein the first and second paths extend between a common upstreamlocation and a common downstream location.
 8. A method as claimed inclaim 6, wherein the first and second paths extend between a commonupstream location and a common downstream location and wherein thecommon upstream clock source is provided at the upstream location.
 9. Amethod as claimed in claim 1, wherein each path carries a respectivesignal stream, the signal stream carried by the first path beingrepresentative of the same content as the signal carried by the secondpath.
 10. A method as claimed in claim 7, wherein the relative time ofarrival of marker signals is measured relative to a common downstreamclock source located at the downstream location.
 11. A method as claimedin claim 1, wherein the difference between the transit times of the twopaths is determined by receiving marker signals from respective entrypoints on the first and second paths, and monitoring the arrival timesof the marker signals.
 12. A method as claimed in claim 11, wherein eachmarker signal includes a time stamp indicative of the time at which thatmarker signal was introduced onto a path relative to a clock source, themethod including the further step of reading the time stamps and takinginto account the time difference between the time at which packets havebeen introduced onto the first and second paths when determining thedifference in the transit times of the two paths.
 13. A method asclaimed in claim 1, wherein each path carries video data.
 14. Areceiving station for receiving data from a sending station sent over atleast a first path and a second path, the paths each carrying respectivemarker signals, the receiving station comprising: a reading stage fordetecting the presence of marker signals, and for monitoring the time ofarrival of marker signals from one path relative to the time of arrivalof marker signals from the other path, and a processing stage fordetermining, in dependence at least in part on the monitored arrivaltimes, the difference in transit times between marker signals travellingalong the first path and the transit time of marker signals travellingalong the second path, an alarm signal being generated if a change inthe difference between the transit times along the first and secondpaths exceeds a threshold value.
 15. A receiving station as claimed inclaim 14, wherein the processing stage is configured to perform thefollowing: (i) calculate the difference between the transit time of amarker on one path and the transit time of another marker on the otherpath; (ii) repeat step (i) for each pair of subsequently receivedmarkers; and (iii) if a change in the difference in transit timeassociated with received pairs of markers reaches the threshold value,generate the alarm signal.
 16. A receiving station as claimed in claim15, wherein an alarm signal is generated only if the threshold value hasbeen reached a predetermined number of times within a time period.
 17. Areceiving station as claimed in claim 14, the receiving and sendingstations being connectable to the network, the sending station beingconfigured, when connected to the network to: send data on a pluralityof paths, the data including marker signals, each marker signalincluding a respective time stamp, the time stamp of a marker signalbeing indicative of the relative time at which that marker signal wastransmitted.
 18. A receiving station as claimed in claim 17, wherein thesending station includes a common clock source, the indication of arelative time included in each stamp being a time measured relative tothe common clock source.
 19. A receiving station as claimed in claim 17wherein the sending station and the receiving station are separated by adistance of more than 10 km.
 20. A receiving station as claimed in claim19, wherein the distance separating the sending station and thereceiving station is at least 100 km.
 21. A receiving station which isconnectable to a plurality of paths, each path having a transit timeassociated therewith for data transport along that path, the receivingstation comprising: means for monitoring the difference between thetransit time of a first path and the transit time of a second path suchthat a change in the difference between the transit times of the twopaths is detected; means for comparing the detected change in differenceagainst a threshold value; and in dependence at least in part on anysuch detected change exceeding the threshold value, means for generatingan alarm signal.